When medieval guards spotted invaders in the distance, they’d pull up the drawbridge and fortify their castle’s defenses. A horse that senses biting flies will flick its tail. The whine of a mosquito’s wings is our cue to start swatting. Plants under attack can’t do any of these things, but that doesn’t mean they’re helpless. Researchers say some tomato plants can sense and defend themselves against encroaching parasitic vines. They published their findings this week in the journal Science. 

Plants’ ability to converts sunlight into nutrients, combined with their inability to run away, makes them juicy targets for bugs, microbes, fungi, and parasitic vines. Experts estimate that parasitic plants alone cause billions of dollars of agricultural damage every year [PDF].

But plants aren’t going down without a fight. Many species have evolved physical defenses like thorns, while others turn to chemical warfare, pumping out terrible-tasting or toxic compounds as soon as they sense a threat. They’re vigilant monitors of chemical signals in their environments, and can even identify invading microbes by their molecules. One plant even farts in the face of danger.

Researchers wondered if the molecular ID technique could work against other types of parasites as well. They decided to test the concept on the tomato plant (Solanum lycopersicum) and one of its would-be adversaries, a vine called Cuscuta reflexa. We say "would-be" because, unlike many of its relatives, S. lycopersicum has somehow found a way to fend off the mooching vine.

C. reflexa on the susceptible tomato relative S. pennellii. Image credit: Eric Melzer

The key moment in host/parasite combat for C. reflexa happens when the vine is still young. Although its seeds are quite hardy, germinating C. reflexa seedlings are vulnerable and will die unless they can find and successfully colonize a plant host within a few days. That colonization can only happen if the parasite can quickly produce syringe-like feeding structures called haustoria that pierce the plant’s cell wall and suck out the nutrients within. To thwart the vine, then, a potential host has to stop the haustoria before they start. And to do that, it needs to know the parasite is there. 

To test the tomato plant's ability to sense its presence, the researchers clipped off small samples of its leaves and dropped them into beakers, to which they also added purified extracts of C. reflexa molecules. They also set up control beakers containing the C. reflexa essence and samples of other, more susceptible plants. Experimenters then took samples of the air inside the beakers and tested it to find out if the alarmed plants were releasing defensive chemicals. 

Sure enough, S. lycopersicum sensed the parasitic vine’s molecules and went into defensive mode. The other plants just kind of … sat there.

Molecular analysis of all the host plants revealed that S. lycopersicum alone contains a receptor protein that the researchers called CUSCUTA RECEPTOR 1, or CuRe1. 

Biologists Vardis Ntoukakis of the University of Warwick and Selena Gimenez-Ibanez of Spain’s Centro Nacional de Biotecnología were not involved in the research, but praised the team’s results. “The identification of CuRe1 represents a major breakthrough in understanding the strategies used by plants to sense danger from diverse origins,” they wrote in a commentary in Science.

They note that other types of parasites also rely on haustoria, and say it makes sense that hosts would use the same mechanisms to keep them all out.

“This work greatly advances our understanding of the mechanisms controlling plant resistance to parasitic plants while at the same time opening up new avenues of research.”

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